Muscles have their own internal clocks and, as a result, function better during the day than they do at night

Practically speaking, this means your workout could yield different fitness results depending on the time of day, with daytime exercise being preferable to nighttime workouts

Your muscles’ circadian rhythm controls exercise response, and your muscles’ ability to use oxygen for energy and adapt to the demands of exercise is most efficient during the day

It’s long been suggested that nighttime workouts should be avoided, in large part because they could disrupt your sleep (although this has since been debated).

New research, however, adds a biological reason why daytime workouts are better — and it has more to do with your body’s circadian rhythm than your ability to fall asleep.

Your body is intricately in tune with daily cycles of light and darkness, right down to your muscles. Research by scientists at Northwestern University in Chicago, Illinois, revealed that muscles have their own internal clocks and, as a result, function better during the day than they do at night.1

What this means, practically speaking, is that your workout could yield different fitness results depending on the time of day, with daytime exercise being preferable to nighttime workouts.

Dr. Joseph Bass, senior study author and chief of endocrinology, metabolism and molecular medicine at Northwestern, said in a news release:2

“Oxygen and the internal clock are doing a dance together inside muscle cells to produce energy, and the time of day determines how well that dance is synchronized … The capacity for a cell to perform its most important functions, to contract, will vary according to the time of day.”

Your Muscles’ Circadian Rhythm Controls Exercise Response

Your muscles’ ability to use oxygen for energy and adapt to the demands of exercise is most efficient during the day. It turns out that muscle cells appear to be most efficient during whichever waking hours are normal for a given organism.

So in the Northwestern study, which used mice, night time turned out to be the better exercise period, which makes sense since mice are nocturnal. Mice exercised on a treadmill at different times of the day and night.

When they exercised at night, their muscles were better able to turn on genes that help with exercise adaptation. The same genes also exist in humans, leading the researchers to believe the same would hold true for people, but in regard to daytime exercise.

The researchers also genetically mutated the circadian clock in isolated muscle fibers, which led to “profound abnormalities.” When you’re resting or exercising gently, your muscles use oxygen to make energy. However, when exercise intensity increases more oxygen is needed so it runs out quickly.

The resulting dip in oxygen triggers proteins called HIFs, which change metabolism and signal your muscles to start using sugar for energy.

This, in turn, leads to increased production of lactic acid. When the muscles’ circadian clock was turned off, however, it impeded this process and prevented exercise from inducing sugar consumption and lactic acid generation.

According to the Northwestern news release, “These findings suggest that better exercise capacity may be tied to specific times of day.”3 The findings may also have implications for diabetes, according to Northwestern:4

“Diabetes is characterized by a failure of muscle to consume glucose, which in turn controls blood sugar levels. Strengthening the muscle clock may provide a new way to eliminate excess glucose and treat diabetes.”

Your Body May Be Hard-Wired to Move at Regular Intervals

Exercising at the appropriate times for your muscles’ internal clock may help them operate more efficiently. In addition, exercise may influence your body’s internal clock and help you recognize the best times to move or be still.

In 2009, for instance, researchers studied the daytime activity patterns of 13 young adults and 13 elderly participants.5 Young people tended to engage in intervals of movement followed by periods of inactivity, primarily during the day, and then resting at night.

This created a “healthy, dynamic circadian pattern,” according to the researchers, which was less robust in the elderly participants, who tended to display more random patterns of movements, including being active during the night and restless during what should have been times of inactivity.

It turns out, however, that exercise — not age — may be the distinguishing factor in helping to regulate a person’s daily movement patterns.

A study in mice (ranging in age from very young to very old) found that when running wheels were provided, the younger mice exercised a lot, developing marked peaks and valleys of activity.6 As in the human study, the older mice were less consistent in their activity patterns.

Once the running wheels were removed, however, the mice quickly fell into more random movement patterns, and the patterns of the younger mice became more like that of the older ones. As noted by The New York Times:7

“By prompting the release of a wide variety of biochemicals in the body and brain … exercise almost certainly affects the body’s internal clock mechanisms and therefore its circadian rhythms, especially those related to activity.

Exercise seems to make the body better able to judge when and how much more it should be moving and when it should be at rest.”

What’s the Best Time of Day to Exercise?

It’s safe to say that exercising in the middle of the night is not ideal for your muscle function or circadian rhythm. But when it comes to exercising during the day, is there a best time? To some extent, the best time to exercise is whenever it will fit into your schedule.

Many people find exercising first thing in the morning is the only way they can fit it into their schedules. Others may be able to fit in a lunchtime workout while some people schedule exercise time in at the end of the day, after work.

You know what your schedule will allow, so don’t get too bogged down with exercising at the “right” time of day — the most important thing is that you’re exercising regularly.

If you’re flexible and your schedule allows you to choose when to exercise, however, there are pros and cons to various times of day. In terms of your circadian rhythm, afternoon exercise appears to be best.

Researchers designed a study comparing the circadian-rhythm effects of exercising at various times of day, using two groups of mice: one healthy group and one group with biologically induced circadian disruptions.8

All of the mice showed positive benefits from exercising, regardless of what time of day they exercised, but the benefits were much higher for the mice whose internal clocks were impaired in the first place.

For those discombobulated mice, after several weeks of running their internal clocks were much more robust, particularly among the mice that exercised in the afternoon. Mice exercising in the late evening showed the least benefit, with some developing even more circadian disruptions, including poor sleep.

So again it appears that exercise has different types of benefits, depending on the time of day it’s performed. From a circadian point of view, it makes sense to see higher benefits from afternoon exercise. Circadian rhythms control your body temperature, which has an impact on your workout.

Your body temperature tends to be a degree or two warmer in the afternoon than in the morning, resulting in better muscle performance and decreased risk of injury. Plus, if you tend to hit that “wall” around 1 p.m. or 2 p.m., going to the gym might be a good way to get over it.

The Benefits of Morning Exercise

There’s also research that suggests morning exercise has benefits of its own. Research shows that 45 minutes of moderate-to-vigorous exercise in the morning may reduce your food cravings, both immediately afterward and throughout the day.9

In that study, morning exercise also resulted in an increase in total physical activity that day, and the women did not compensate for the energy expenditure by eating more later in the day, which suggests morning exercise may help you keep moving even after your workout, which is another key to optimal health.

Morning exercise also makes it easy to exercise while fasting, which will amplify the benefits you receive. Research has shown that exercising on an empty stomach is useful for preventing both weight gain and insulin resistance. One of the explanations for this is that your body’s fat-burning processes are controlled by your sympathetic nervous system (SNS), and your SNS is activated by exercise and lack of food.

The combination of fasting and exercising maximizes the impact of cellular factors and catalysts (cyclic AMP and AMP Kinases), which force the breakdown of fat and glycogen for energy. This is why training on an empty stomach will effectively force your body to burn fat.

If you have trouble exercising on an empty stomach, you can consume 20 grams of a fast-assimilating protein like a high-quality whey protein concentrate 30 minutes before your workout.

Evening Exercise May Not Be so Bad After All

I generally discourage vigorous exercise in the evenings, but if you’re a devoted evening exerciser there may not be a reason to force a change. A study published in 2011 found that when people exercised vigorously for 35 minutes right before bed they slept just as well as on nights when they didn’t exercise.10

Another study, a poll by the National Sleep Foundation (NSF), found that 83 percent of people said they slept better when they exercised (even late at night) than when they did not.11

More than half of those who exercised moderately or vigorously said they slept better on workout days than non-workout days, and just 3 percent of late-day exercisers said their sleep quality was worse when they exercised than when they did not. The NSF concluded that exercise is good for sleep, regardless of the time of day it’s performed.

If you’re not sure which time of day you prefer to exercise, you can do some experimentation of your own. Perhaps try a month of exercising in the morning, followed by a month of exercising in the afternoon, as your schedule allows. With the increasing realizations of the importance of circadian rhythms on human health, and the suggestion that afternoon exercise may be best for optimization of same, it may be that afternoon exercise stands out above the rest.

Ultimately, however, you should listen to your body and let it be your guide in choosing what time of day works best for you. For many people, the “best” time to exercise may also change day-to-day to accommodate schedules, and that’s fine too.